Take a thick black marker and mark a wide strip on the inside of your front rims from the inner edge inward for 1/2 inch. Then reproduce the rubbing sound and check to see if there is a line through the marker. If there is, then you should be able to find the ink on whatever part is rubbing.
If none of the ink is rubbed off either rim, then the problem is most likely coming from the drive shaft rubbing against the inner lip of the hex hub(s), or the round ball end of the drive shaft being warped and rubbing against the inside of the hex hub(s).
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